Car performing bad and noisy

I had it serviced and just gone through its MOT with no problems. Car very lumpy when in idle, when driving very jerky hardly any power and engine management light come and flashes also traction control light. I have got a code reader diagnostic machine plugged it in and it came up with code P0304. which states cylinder four misfire detected. took coil pack off and noticed the forth rubber was all split. checked spark plugs which did not look too bad so I changed coil pack and the code no longer shows. but, still shocking when running. Any idea what this could be? Please.Thanks.